1. Field of the Invention
Various embodiments of the invention pertain to sealed chip packaging. More particularly, one embodiment of the invention pertains to a system, device, and method for incorporating getters into hermetically sealed silicon microdevices, in particular silicon gyroscope chips.
2. Description of Related Art
Certain silicon microdevices, such as vibratory gyroscopic rate sensors, typically require very low gas damping (high Q) to operate. This is often accomplished by operating the device in a vacuum, reducing gas damping to a low level. At the present, devices required to be operated in vacuum are typically mounted in packages that can then be sealed in vacuum. It is also important to maintain that same low pressure over the operating period of the gyroscope, since a change in pressure over time can cause performance errors. Silicon microdevices can be made and sealed hermetically. This is usually accomplished at the wafer level by such methods as silicon direct bonding, anodic bonding, metal sealing, glass frit bonding, or polymeric adhesion. In most applications for wafer level packaging or encapsulation, microdevices are sealed at atmosphere pressure. Although microdevice cavities can be sealed in this fashion in vacuum, the vacuum level quickly degrades due to outgassing, permeation, and/or virtual leaks.
Incorporation of one or more active getters within the cavities of a microdevice chip serves to stabilize the vacuum level, as gasses are constantly sorbed to create a steady state pressure. To incorporate a gettering capacity within a cavity, a getter must be activated or fired (depending on the type) at high temperatures, usually at or higher than about 400° C. This must be done in vacuum or an inert gas (e.g., helium, neon, argon, etc.) and in situ immediately prior to sealing, or the getter loses its capacity due to re-sorption of active gasses (hydrogen, oxygen, nitrogen, etc.). Thus, to incorporate getters into cavities defined by bonding silicon wafers together, the getters must be positioned in the cavities before the bonding process. They must then be activated within a bonding chamber that includes vacuum and a bonding mechanization to bring wafers together to form the bond. The bonding process can include parameters such as pressure, heat, electrical current, or any other parameter that would normally be used to bond wafers with the selected technique.
An additional requirement for fabrication of certain microdevices, such as a silicon gyroscope chips, is that the wafers be precisely aligned, to within several microns or less, at the time of bonding. In effect, this requires that getters be inserted into a first wafer or wafer assembly prior to alignment of a second wafer or wafer assembly so as to effect and maintain the required level of alignment precision throughout the bonding process.
Typically, fabrication of such microdevices also requires the stable positioning and bonding of the getter itself to the interior of the cavity. Securing the getters to the interior cavity prevents the getters from moving within the cavity during operation and interfering with device motion or electrical integrity.
Another common requirement for fabrication of such microdevices is the application of a high vacuum, with pumping, during the getter activation time and immediately thereafter until the wafers are brought together and sealed.